1. Field of the Invention
The present invention generally relates to an optical transmission device. More particularly, the present invention relates to a mounting structure or a printed board unit for optical transmission, and a mounting method of mounting a surface light emitting (light receiving) semiconductor element, which emits or receives a laser beam used for optical transmission, on an optical wave-guide substrate.
2. Description of the Related Art
The following prior art is known.
Japanese Unexamined Patent Publication No. 9-26530 discloses the following technique. In an optical module in which optical elements of surface light emitting (light receiving) type and optical fibers are arranged on a plane of the same mounting substrate, the optical module having an optical coupling system, in which an optical path is converted on a side of a stationary groove of the optical fibers, in order to stabilize the characteristic and reduce the manufacturing cost, a surface light emitting element (for example, an LED) is first mounted on a sub-substrate via a bump, and then the sub-substrate is mounted on the mounting substrate via another bump.
The prior art disclosed in Japanese Unexamined Patent Publication No. 9-26530 has the following problems. A surface light emitting element (for example, an LED) is first mounted on a sub-substrate via a first bump, and then this sub-substrate is mounted on a mounting substrate via a second bump. Therefore, the distance between the sub-substrate and the mounting substrate is determined by the size of the second bump. Therefore, when the size of the second bump fluctuates, the interval between the sub-substrate and the mounting substrate for each optical module fluctuates and, further, the distance between the light emitting (light receiving) element and the optical fiber fluctuates. Accordingly, the light emitting or light receiving characteristic is changed, so that the performance of the individual optical module can not be stabilized. Further, there is a problem in which a gap formed between the light emitting (light receiving) element and the optical wave-guide substrate 1 can be clogged with dirt and dust.
In the conventional system shown in FIG. 1, the following problems may be encountered. A gap corresponding to the height of the solder bump 6 is necessarily made between the light emitting (light receiving) element and the substrate. Therefore, the distance between the element light emitting (light receiving) section 20b of the element and the optical wave-guide 10 right below the substrate is extended.